For many coating applications such as automotive coatings, aerospace coatings, industrial coatings and architectural coatings, dark colors, such as black and dark blue are particularly desirable for aesthetic purposes. However, dark colored vehicles and buildings (or other enclosed spaces) are susceptible to absorption of infrared (IR) radiation. These dark colored structures reflect insignificant amounts of IR radiation. As a result, the structures exhibit increased temperatures and become quite hot, particularly on sunny days, rendering them uncomfortable for their occupants. In addition, such vehicles or buildings are then more expensive to operate since higher levels of air conditioning are required to maintain them comfortably, as compared to structures having lighter colors with high reflectivity, such as vehicles that are coated with white or silver coating compositions.
Dark colored coating compositions have conventionally used carbon black as a pigment. Carbon black absorbs a broad spectrum of visible radiation and provides a desirable dark black color (jet black) when used in a coating composition. However, this broad spectrum also includes radiation outside the visible range. Accordingly, black coatings containing carbon black have a tendency to increase in temperature as the energy of the non-visible radiation is absorbed along with the visible radiation.
One solution for avoiding the thermal load experienced by black coatings containing carbon black has been to provide a two layer system. An upper layer contains an organic black pigment or a mixture of organic pigments that absorb visible radiation to exhibit a dark color but which are substantially transparent to IR radiation, and a lower layer includes a composition that reflects IR radiation. While the lower IR reflecting layer reflects IR radiation and minimizes the temperature increase of the coating system, the overlying organic pigment layer contains pigments that substantially scatter light compared to well-dispersed carbon black pigment. Accordingly, the organic black pigment layer does not achieve a true black color and may appear grey or brown.
In another approach, IR reflecting compositions include a black pigment as a colorant and a reflective pigment to reflect the IR radiation. Again, such coating compositions do not typically exhibit a deep color such as jet black.